Commonly known methods for producing an expanded polyamide resin molding include a method using a chemical foaming agent (a chemical foaming method). The chemical foaming method is a method that comprises mixing a raw material resin and an organic foaming agent that decomposes on heating to generate gas and then heating the mixture to temperature equal to or higher than the decomposition temperature of the foaming agent. For example, in Patent Document 1, a polyamide terpolymer is used and an expanded polyamide molding having a specific gravity of 1.2 has been obtained by using a chemical foaming agent. However, this expanded polyamide molding is low in expansion ratio and therefore has failed to satisfy weight reduction sufficiently.
As a method for producing an expanded polyamide resin molding other than methods using a chemical foaming agent, Patent Document 2 has proposed a method that comprises making a polyamide molding absorb carbon dioxide beforehand, then heating it in a post step, thereby obtaining an expanded polyamide molding having an expansion ratio of 2. However, it cannot be said that the expanded polyamide molding produced by this method has not been reduced in weight sufficiently and the method has a disadvantage that it is complicated and low in productivity because the molding step and the expanding step are substantially separate steps.
Moreover, Patent Document 3 has disclosed a method for producing an expanded polyamide molding by dissolving a supercritical fluid of nitrogen or carbon dioxide in a molten resin, followed by injection molding. However, this method achieves an expansion ratio as low as 1.25 and therefore has failed to realize sufficient weight reduction.
On the other hand, Patent Document 4 has disclosed a method for obtaining an expanded molding with a fine average cell diameter using a polystyrene resin, but the method has a disadvantage of lacking versatility because not only a common injection molding machine but also a specially designed injection plunger and a specially designed injection apparatus are needed in order to obtain an expanded molding of interest. Moreover, the expanded molding reported in this document is only one prepared using a polystyrene resin, which can be expansion-molded relatively easily in an existing expansion molding method, and even if this method is applied to a polyamide resin, which is difficult to be expansion-molded, a desired expanded molding is not readily available.
Moreover, Patent Document 5 has proposed a method in which when a molten resin filled into a mold has reached a certain viscoelastic state during a cooling process, a core-side mold is moved in the mold opening direction and simultaneously a critical inert gas is injected directly into the resin in the mold, so that an expanded molding is obtained. However, it has been difficult to form uniform foam cells by this method because a crystalline polyamide high in solidification rate can maintain a proper viscoelastic state in a short period of time.
As described above, a polyamide is a resin superior in heat resistance and mechanical characteristics and has been expected to be used in the form of various resin moldings such as automotive components and household electrical appliance components. However, since the melt viscosity characteristics of a polyamide are not suited for expansion molding, it has been difficult to obtain, by a simple molding method, an expanded polyamide resin molding having a uniform expanded layer with an expansion ratio high enough for attaining sufficient lightweightness.
Incidentally, in recent automobile design aiming at reduction in fuel consumption, a reduction in weight by replacing a metal component with a resin molding component is a significant challenge. However, automotive components such as an engine cover, a cylinder head cover, and a transmission cover, are required to have high heat insulating properties and high oscillation resistance as well as lightweightness, and there are not so many resin moldings that satisfy such requirements. Structures for attaining weight reduction and heat insulating properties include expanded structure bodies. However, polypropylene, polystyrene, polyethylene, and the like, which are low in heat resistance, cannot satisfy load resistance required as moldings because those materials themselves are degraded or softened rather than heat insulating properties under the environment of use of 100° C. or higher. Thus, expanded moldings of a polyamide resin with superior heat resistance have been expected to be used as alternative resin molding components for automotive components. In use as automotive components, however, heat insulating properties and oscillation resistance are required in addition to lightweightness which is usually possessed by expanded moldings as described above.